1. Field of the Invention
This invention relates to a method for producing a bipolar transistor fabricated in a semiconductor IC.
2. Description of the Prior Art
It is required more and more that a bipolar transistor in an IC operates at a high speed. For this purpose, it is known to make the distance between an emitter region and an extrinsic (or outer) base region as short as possible. To this end, the emitter region is formed in an intrinsic base region in a self-aligned matter with the extrinsic base region.
More specifically, as shown in FIG. 1, a semiconductor body is prepared, which includes a p-type semiconductor substrate 1, an n-type epitaxial layer 3, an n-type buried layer 2 formed between the substrate 1 and layer 3, a collector contact region 7, and a field insulating film 4 selectively formed on the layer 3. After exposing a surface portion of the layer 3 in which the base and emitter regions are to be formed, a polysilicon layer 16 doped with impurities and an silicon oxide film 8 are formed in that order over the entire surface of the body. The oxide film 8 is then patterned to be removed except portions thereof to define a base electrode. The polysilicon layer 16 is thereafter selectively removed in an anisotropically etching manner by using the patterned oxide film 8 as a mark. The base electrode 16 is thus formed.
It is noted, however, that the formation of the base electrode 16 is accomplished with much difficulty of stopping with a high precision at the interface between the portion of the electrode 16 and n-type epitaxial layer 3. For this reason, it is unavoiable that part of the surface portion of n-type epitaxial layer 3 in which the emitter region is to be formed is excessively etched. Such an excessive etching of the layer 3 is made larger in order to completely remove the parts of the polysilicon layer 16 covering the stepped edge portions 17 of the field oxide layer 4, as shown in FIG. 1.
Thereafter, a heat treatment is carried to diffuse the impurities in the layer 16 into the layer 3. Consequently, as shown in FIG. 2, an extrinsic base region 14 is formed in the layer 3 as a collector region. Ion-implantation is then carried out to form an intrinsic base region 13, followed by forming a sidewall oxide film 9 by the well-know technique. A polysilicon layer 11 doped with impurities are thereafter formed, and an emitter region 15 is then formed by duffusing the impurities from the layer 11 into the base region 15.
As described hereinbefore, however, due to the excessive overetch of the surface portion of the layer 3, the extrinsic and intrinsic base regions 14 and 13 are connected with a slight contact, as shown in FIG. 2. For this reason, the base resistance is remarkably increased to degradate the electrical characteristics of the transistor thus fabricated.